1. Field of Use
The present invention relates generally to the field of communication systems. More particularly, the present invention concerns control systems that have remote sensors that transfer status information about a process condition and diagnostic information about the sensors themselves. Specifically, a preferred embodiment of the present invention is directed to control systems that have remote sensors that transfer diagnostic information about the remote sensors themselves to a central control station through modulation of a process status signal. The present invention thus relates to a communication system of the type that can be termed diagnostic signaling.
2. Description of Related Art
Heretofore, it was known in the prior art to provide, as part of a control system, remote sensors that communicate their "on/off" state through a pair of wires in which a current exists. Referring to FIG. 1A, this type of remote sensor permits a relatively large current flow to represent a first of the two "on/off" states and a relatively small current to represent the second of the two "on/off" states.
A previously recognized problem has been that a remote sensor can malfunction. When such a malfunction occurs, the current flow from the remote sensor may go to zero and no longer accurately communicate the "on/off" state of the remote sensor. However, the control system to which the remote sensor is connected may misinterpret zero current from the remote sensor to be a normal indication of the relatively small current that represents the second of the two "on/off" states. Alternately, a failure may connect the supply voltage to the sensor output and this would be indistinguishable from the "on" state. What is needed therefore is a method of transmitting encoded diagnostic information from a remote sensor to a central control station. Heretofore, this requirement has not been fully met without incurring various disadvantages.
One unsatisfactory previously recognized solution to the problem of transmitting diagnostic information from a remote sensor was to separately cable the diagnostic information from the remote sensor to a receiving computer or logic controller located in the central control station. Although remote sensors cabled in this way are capable of diagnostically monitoring their own operation by a variety of specific structures and communicating the diagnostic information to the central control station, a disadvantage of this previously recognized solution is that the high cost of separately cabling the diagnostic information from the remote sensor to the receiving computer or logic controller frequently prohibits the economical use of such diagnostic information. Further, this previously recognized solution also has the disadvantage of inherently increasing the susceptibility of the system to malfunctions due to the necessary increase in the mount of wiring.